pft_ping: improve IPv6 address comparison

Don't use string comparisons, use socket.inet_pton() instead. This avoids
confusion when there are different ways to spell the same IP addres.
e.g. 64:ff9b::c000:202 and 64:ff9b::192.0.2.2 are two representations of the same
address.

Sponsored by: Rubicon Communications, LLC ("Netgate")